This invention relates to a novel method for preparing filter-coated phosphor particles.
Certain technical terms are used herein as defined by the International Union or Pure and Applied Chemistry and published in Pure and Applied Chemistry, 1972, Vol. 31 No. 4, pages 605 to 613. The term filter includes pigments and is defined as a material which has a selected visual color (not black or white) to the human eye when viewed in white light. As used herein, a red filter or red pigment appears to be red and a blue filter or blue pigment appears to be blue to the human eye when viewed in white light.
Filter-coated phosphor particles and their use in viewing screens for cathode-ray tubes have been described previously; for example, in U.S. Pat. Nos. 3,308,326 to S. H. Kaplan, 3,875,449 to W.H. Byler, and 3,886,394 to S. A. Lipp. As disclosed in these patents, filter material may completely cover the phosphor surfaces, or may only partially cover the surfaces of the phosphor particles. The filter material is prepared separately and then deposited onto the surfaces of the phosphor particles. The resulting filter-coated phosphor particles may then be used in the same general manner as uncoated phosphor particles to prepare viewing screens for cathode-ray tubes. It has been found, however, that particles of filter material become separated from their phosphor particles during screen preparation. This separation occurs, for example, when the filter-coated particles are used in the slurry-direct photographic printing method to make multicolor screens where the separation may result in filter particles of one color contaminating light-emitting elements of another color. When filter particles become separated from their phosphor particles, it is more difficult to recycle excess slurry and to reuse salvaged phosphor because of the uncertainty of the content of the filter material therein.